RS-485 continues to be the workhorse in industrial networking, as it has been for more than three decades. Advances in process technology and design have reduced transceiver quiescent current (or supply current without loading) from 40 mA down to 1 mA. The ongoing trend in energy preservation, however, encourages system designers to save even more power by reducing bus currents.
The only possible solution appears to be the removal of the termination resistors that enable dc currents to flow during the steady state of a bit time interval. The three commonly applied alternatives to parallel termination are operating the bus without termination, implementing an ac-coupled termination, or simply clamping the bus lines to the supply potential, also known as diode termination (Fig. 1).
Each one of these techniques tampers with the parallel termination concept suggested in the RS-485 standard and causes unforeseen risks and consequences with regards to signal integrity, electromagnetic interference (EMI), and network robustness. Designers, then, should determine whether the small amount of power saved outweighs the need for standard compliance.
Bus Current Requirements
The RS-485 specified driver test circuit requires a driver’s differential output voltage to be at least 1.5 V across a 60-Ω differential load (2 x 120 Ω in parallel) while driving a common-mode load of 32 unit loads (375 Ω) under a maximum common-mode voltage variation of –7 V to +12 V (Fig. 2).
The minimum current drive requirements for both, the sourcing (IOH) and sinking (IOL) driver outputs, are determined via:
VOD = the minimum differential driver output voltage
VOS = the driver output offset voltage, which is approximately half the supply voltage, VOS = VCC/2
VCM = the common-mode voltage range from –7 V to +12 V
RD = the differential resistance resulting from paralleling the two 120-Ω termination resistors
RCM = the common-mode impedance presented by the maximum of 32 unit loads
The V-I diagram in Figure 1 shows that for a 5-V transceiver bus, current can assume up to ±53 mA at both common-mode extremes.
Removing the termination resistors eliminates the differential current of ID = 1.5 V/60 Ω = 25 mA. The common-mode currents into the transceiver inputs, though, continue to flow. It is therefore impossible to eliminate the bus current entirely. The only option to further reduce common-mode currents is by choosing transceivers with less unit-loading (UL), such as 1/8 UL devices. These transceivers posses input impedances that are eight times higher than their legacy 1 UL predecessors.
Operating the bus without termination causes standing waves to occur as the incident wave is reflected entirely at the bus end. Traversing back to the driving source, the reflected waves mix with other incident waves, yielding standing waves for signal frequencies whose quarter wavelengths, or multiples thereof, equal the length of the data link (Fig. 3).
Depending on the location of the wave minima (nodes) and maxima (antinodes), their impact on an individual transceiver varies between locations. A driver close to an antinode drives into a high impedance line and, therefore, transfers insufficient energy into the bus. A driver close to a node is driving into a low-impedance or shorted line. The resulting high output current can trigger the driver current limiter at around 250 mA.
Receivers located at antinodes can be damaged due to excessively large receive signals exceeding the receiver input common-mode range. Receivers close to nodes experience insufficient signal strength and are highly susceptible to noise and interference.
With all of these events, data errors occur either due to the transmission or the reception of wrong data. So to operate an unterminated bus without data errors, the signal frequency or data rate must be drastically reduced, which means extending the signal wavelength beyond the n • λ/4 condition to prevent standing waves from occurring. A rule of thumb suggests the one-way propagation time to be one-tenth of the bit time interval.
Figure 4 compares the maximum data rates for a terminated bus at 5% and 10% jitter and the data rates for an unterminated bus over various cable lengths. As can be seen, operating an unterminated bus requires a steep drop in data rate.
For example, a 100-m bus properly terminated allows for data rates of up to 5 Mbits/s. The same bus unterminated won’t be able to support rates above 200 kbits/s without compromising signal integrity. A lower data rate, however, means less data throughput, which, in industrial automation where networks must run constantly without any down time, can mean less production efficiency and lower profitability.
The ac-coupled termination intends to provide bus termination during signal transitions only while preventing dc current flow during the steady state (Fig. 1, again). For this purpose, a small capacitor is switched in series with a termination resistor. The value of the termination capacitor is calculated via:
tDL = tDU • √((1 + CD – XCVR)/(CD – cable)) is the signal delay of the loaded line
tDU = L/(c • v) is the signal delay of the unloaded line
CD – XCVR = n • CID/L is the distributed transceiver capacitance
CD – cable = the distributed cable capacitance
CID = the transceiver differential input capacitance
L = the bus length (ft)
c = the speed of light (9.8 • 108 ft/s),
v = the signal velocity in the cable as a factor of c
Equation 3 indicates that for a given type of bus cable, CT depends on the cable length and the number of transceivers connected to the bus. Thus, any modifications to the data link such as adding transceivers or extending the bus length will require a different CT to accomplish the desired reduction in steady state current. The maximum data rate for a given R-C time constant is determined via:
DR ≤ 0.1/(RT • CT) (4)
which confirms that for a given CT value, the data rate for an ac-terminated bus needs to be drastically reduced to make this current reduction work effectively. For higher data rates, the capacitor reactance drops and current reduction becomes ineffective, quickly approaching current levels of the parallel termination.
In addition to the previously mentioned drawbacks of reduced data throughput, another issue is the increase in inter-symbol interference (ISI) due to higher capacitive loading caused by the termination capacitors. Higher ISI means an increase in bit errors and a faster closure of the eye-pattern, all of which are counterproductive to a robust data transmission link.
The diode termination (Fig. 1, again) really doesn’t terminate anything but rather clamps the signal pair via Schottky diodes to the positive and negative supply rails. Reflections on the bus are clamped as long as their voltages exceed the supply rails by the forward voltage drop of a diode. Reflections below that level, however, continue to proceed back to the driver source.
Allegedly, diode termination doesn’t require any computation and can be applied at several bus locations. These claims, however, don’t present advantages over parallel termination. Because the RS-485 standard already suggests the use of 120-Ω termination resistors, no computation is required either. Also, parallel termination only requires placing a resistor at each bus end. Unlike diode termination, where reflections continue to exist despite clamping action, parallel termination truly eliminates reflections.
Diode termination indeed has some major drawbacks. During clamping action, when a diode conducts, a bus line is directly connected to a supply rail, causing huge current spikes on the bus. These spikes contribute to a drastic increase in electromagnetic interference (EMI) as well as force the driver into current limiting, which in turn distorts the transmitted signal.
Another drawback is the clamping to the supply rails. Long-distance interfaces must operate reliably over a wide common-mode voltage range. For RS-485, this range is 7 V beyond both supply rails. At these high common-mode voltages, however, diode termination will clamp data signals to the supply rails and, therefore, prevent any reliable data transmission from happening.
As with all other non-parallel termination techniques, diode termination suffers from the requirement of a drastically reduced data rate to ensure reflections occur and dampen within the first 10% of the bit time.
Assuming a continuously operating network of 5-V transceivers with a maximum bus current of 60 mA, the maximum bus power is 300 mW. Having presented the pitfalls and drawbacks of the three alternatives to parallel termination, simply to save some 300 mW, is it really worth the risk of designing a non-reliable, non-compliant network?
If you really want to save power, replace the standard 60-W light bulb in your office for an energy-saving one. This should give you a 50% power savings right away. With regards to network design, however, choose low-power transceivers to save quiescent power right from the beginning. Then, apply the 120-Ω parallel termination suggested in the RS-485 standard to maintain a standard compliant network that ensures high data rates over long distance, low EMI, and high common-mode robustness.
- Termination Techniques for High-Speed Busses, California Micro Devices, Application Note AP204, 2001
- The RS-485 Design Guide, by Thomas Kugelstadt, Texas Instruments, SLLA272B, May 2008
- The RS-422 and RS-485 Standards Overview and Systems Configurations, by Kevin Zhang, Clark Kinnaird, and Thomas Kugelstadt, Texas Instruments, SLLA070D, May 2010